## Abstract

Microchannels are being considered in many advanced heat transfer
applications including automotive and stationary fuel cells as well as
electronics cooling. However, there are a number of fundamental issues from the
heat transfer and fluid mechanics perspectives that still remain unresolved. The
present work focuses on obtaining the fundamental heat transfer data and
two-phase flow patterns present during flow boiling in microchannels. An
experimental investigation is performed for flow boiling using water in six
parallel, horizontal microchannels with a hydraulic diameter of 207 μm. The
ranges of parameters are: mass flux from 157 to 1782 kg/m^{2}s, heat
flux from 5 to 930 kW/m^{2}, inlet temperature of 22°C, quality from
sub-cooled to 1.0, and atmospheric pressure at the exit. The corresponding
single-phase, all-liquid flow Reynolds number range at the saturation conditions
is from 116 to 1318. The measured single-phase, adiabatic pressure drop agreed
with the conventional theory within the experimental error. The experimental
single-phase Nusselt number was found to be between the constant heat flux and
the constant wall temperature boundary conditions, corresponding to $NuH$ and $NuT$ respectively. The flow visualization demonstrates that the flow reversal
condition in parallel flow channels is due to bubble nucleation followed by its
rapid growth. In addition, the dry-out condition is observed, showing a change
in the contact angles of the liquid-vapor interface. The local flow boiling heat
transfer coefficient exhibits a decreasing trend with increasing quality. A
comparison with the nucleate boiling dominant regime of a flow boiling
correlation shows good agreement, except for the large peak in two-phase heat
transfer coefficient observed at the onset of nucleate boiling.

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